While it is possible to construct the rules around the notes of a scale that belong in a key, the key itself really makes sense when paired with music. It is the music that determines the key signature that would be "correct".

One way is to play the music and see how the melody resolves to a "home" note. That would usually be the "correct" key signature.

The other way is to let the composer decide how they intended their composition to be interpreted.

At the end of the day, this is "music theory" not "key theory" or "note theory". So let the music guide you.

EDIT: Once you figure out the tonal center of the song, then the notes to use for the scale must have (and this is always true) ALL the notes "A, B, C ... G" in some sequence which tells you whether to use the flat or the sharp. For example, you should not get B, Dflat, Eflat... because you skipped C. It would be B, Csharp, Dsharp.